RFR: 8377907: (process) Race in ProcessBuilder can cause JVM hangs

[Thomas Stuefe]

When starting child processes from Java, we bootstrap the child process after fork and before exec. As part of that process, up to five pipes are handed to the child: three for stdin/out/err, respectively, and two internal communication pipes (fail and childenv).

If, concurrently with our invocation of `ProcessBuilder.start()`, third-party native code forks a child of its own, the natively forked child carries copies of these pipes. It then may keep these pipes open. This results in various forms of communication errors, most likely hangs - either in `ProcessBuilder.start()`, or in customer code. 

In the customer case that started this investigation, `ProcessBuilder.start()` hung intermittently when using a third-party Eclipse library that happened to perform forks natively.

The JVM has no full control over what happens in its process, since we allow native code to run. Therefore, native forks can happen, and we have to work around them. 

The fix makes sure that the pipes we use in ProcessBuilder are always tagged with CLOEXEC. Since forks are typically followed by execs, this will close any file descriptors that were accidentally inherited.

### FORK/VFORK mode

Here, it is sufficient to open all our pipes with O_CLOEXEC.

The caveat here is that only Linux offers an API to do that cleanly: `pipe2(2)` ([1]). On MacOS and AIX, we don't have `pipe2(2)`, so we need to emulate that behavior using `pipe(2)` and `fcntl(2)` in quick succession. That is still racy, since we did not completely close the time window within which pipe file descriptors are not O_CLOEXEC. But this is the best we can do.

### POSIX_SPAWN mode

Creating the pipes with CLOEXEC alone is not sufficient. With `posix_spawn(3)`, we exec twice: first to load the jspawnhelper (inside `posix_spawn(3)`), a second time to load the target binary. Pipes created with O_CLOEXEC would not survive the first exec.

Therefore, instead of manually `dup2(2)`'ing our file descriptors after the first exec in jspawnhelper itself, we set up dup2 file actions to let posix_spawn do the dup'ing. According to POSIX, these dup2 file actions will be processed before the kernel closes the inherited CLOEXEC file descriptors.

Unfortunately, macOS is again not POSIX-compliant, since the macOS kernel can close CLOEXEC file descriptors before posix_spawn processes them in its dup2 file actions. As a workaround for that bug, we create temporary copies of the pipe file descriptors that are untagged with CLOEXEC and use them as sources for posix_spawn dup2 actions. Of course, these file descriptor copies would again leak to concurrently forked processes. But since these are only temporary copies, that is benign.

### Code changes

We replace the manual file descriptor dup'ing in jspawnhelper with posix_spawn file actions. That means we can scrap everything associated with that: it is no longer necessary to pass file descriptors via command-line arguments to jspawnhelper or via the `ChildStuff` structure.

However, I tried to keep code reshuffling minimal and this patch as small as possible. While working on this code, I had a number of ideas for streamlining and improvements, but I will hold that off for future RFRs. This patch aims to be small and easily backportable.

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Testing:

The patch adds three new tests:

1) `ConcNativeForkTest` is a reproducer for the "native concurrent forks cause hangs in ProcessBuilder.start()" problem. It is pretty good at provoking hangs in unpatched JVMs. However, it is not suited for automated CI/CD environments since it essentially fork-bombs itself. Therefore, I marked the test as "manual".

2) `PipesCloseOnExecTest` is a modified variant of `ConcNativeForkTest` that, instead of executing concurrent native forks, continuously checks that no file descriptors pop up untagged with CLOEXEC during a `ProcessBuilder` invocation. This test is fast and reliable, but it works only on Linux, since only with `pipe2(2)` we can completely close the !CLOEXEC time window. On MacOS and AIX, the test correctly spots untagged file descriptors since we still have the small time window between `pipe(2)` and `fcntl(2)`.

3) Finally, `FDLeaks` is a complementary test that checks that we don't accumulate unclosed file descriptors in ProcessBuilder.start().

- All tests in java/lang/ProcessBuilder were run, and all issues fixed. In particular, I tested:
	- Linux x64 with glibc
	- Linux x64 with muslc (Alpine)
	- MacOS Arm64
- Ran jdk + hotspot tier1 on Linux x64
- Tests on AIX are ongoing at SAP
- GHAs ran through successfully

[1] https://linux.die.net/man/2/pipe2

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Commit messages:
 - wip
 - copyrights
 - new test
 - start

Changes: https://git.openjdk.org/jdk/pull/29939/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29939&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377907
  Stats: 1250 lines in 16 files changed: 1123 ins; 54 del; 73 mod
  Patch: https://git.openjdk.org/jdk/pull/29939.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29939/head:pull/29939

PR: https://git.openjdk.org/jdk/pull/29939